In recent years, electrification of a car has advanced, and a number of electronic devices mounted on a vehicle tends to increase year by year. Further, a control function required for each electronic device has been more sophisticated accordingly. For the electronic device incorporating an electronic circuit, it is essential to protect an electronic component mounted on a circuit board from an influence of heat.
Particularly, for the electronic component of the electronic device used around an engine room of the car (environment where peripheral temperature is 90° C. or more), improvement of thermal insulation properties suppressing transmission of heat transmitted from a periphery of the engine room is also required in addition to improvement of heat dissipation properties dissipating heat generated by itself.
Conventionally, as measures against a temperature rise of such an electronic component, for example, the following proposals have been made to improve the heat dissipation properties and the thermal insulation properties against heat from heat generator formed with another heat generating component (hereinafter, “heat generator formed with another heat generating component” is simply referred to and described as “heat generator” as appropriate).
PTL 1 discloses a mold motor in which a circuit board mounted with an electronic component for driving a motor and a stator of a magnetic circuit unit serving as a heat generator are integrally molded of thermosetting resin. Also, the mold motor in PTL 1 has a thermal insulation structure in which a thermal insulation layer having low thermal conductivity is provided between the circuit board and the stator.
Further, PTL 2 discloses a configuration in which a partition wall is provided between a circuit board and a stator serving as a heat generator of a motor similar to the above-described motor. Also, the motor in PTL 2 has a thermal insulation structure in which a rubber sheet with thermal insulation properties is stuck on a surface on a circuit board side of this partition wall or in which a paint with thermal insulation properties is applied onto the surface.
In PTL 1 or 2, by providing such a thermal insulation structure, transmission of heat from the stator to the electronic component on the circuit board is prevented, and a temperature rise of the electronic component is avoided.
Moreover, PTL 3 discloses, in a motor, means of sticking a heat reflection material on a partition wall similar to the above-described partition wall, of forming a plated layer on the partition wall, of forming the partition wall into a mirror finished metal plate, or the like. Also, the motor in PTL 3 attains thermal insulation of an electronic device by reflecting radiant heat of a heat generator by such means.
Further, PTL 4 discloses an electric water heater having a configuration in which a thermal insulation material is closely adhered to a water storage container to keep stored water warm. Also, in the electric water heater in PTL 4, a solidified thermal insulation material is used as this thermal insulation material. The thermal insulation material has excellent thermal insulation performance so that thermal conductivity of the material is lower than or equal to thermal conductivity of stationary air. Further, in PTL 4, silica xerogel known to have extremely high thermal insulation performance among non-vacuum thermal insulation materials is proposed as one example of a raw thermal insulation material of such a thermal insulation material.
However, the mold structure as in PTL 1 cannot sufficiently obtain a heat radiation effect by the mold resin, particularly, in a case where the mold structure is used in a condition where peripheral temperature is high, such as a periphery of an engine room. Also, even when the layer having the low thermal conductivity, such as an air layer, is provided, since thermal conductivity of the mold resin itself is not so low, the mold structure receives heat from the magnetic circuit unit serving as the heat generator via the mold resin. Temperature of the electronic component may thus exceed heat resistance temperature of the electronic component.
Further, as in PTL 2, in the configuration in which the rubber sheet with the thermal insulation properties is stuck to the partition wall provided between the electronic circuit unit and the heat generator, since thermal conductivity of the rubber sheet (about 0.2 W/m·K) is generally larger than thermal conductivity of the air (about 0.026 W/m·K), a thermal insulation effect is small. With this configuration, it is necessary to increase a thickness dimension to enhance the thermal insulation effect, and the size of the device may become larger. On the other hand, in the configuration in which the paint with the thermal induction properties is applied, even when a paint having high viscosity and capable of applying a thick film is used, multiple layers of the paint must be applied to enhance thermal insulation performance, and it takes much time. Moreover, since the viscosity of the paint is high, a special coating device is required, and further, large capital investment is required. Also, when a paint with low viscosity is used, film cannot be made thick, and the required thermal insulation performance cannot be obtained.
Moreover, as in PTL 3, in the method in which the radiant heat from the circuit board or the magnetic circuit unit is reflected, the heat is returned to a place where temperature is originally desired to be lowered by heat radiation. Accordingly, the method may cause a decrease in reliability or in performance due to the temperature increase.
Further, as in PTL 4, in a case of the thermal insulation material solidified with the silica xerogel, the silica xerogel is solidified using a powdered raw thermal insulation material and an aqueous binder, thereby forming a molded body serving as the thermal insulation material. However, the thermal insulation material formed in this way has a problem of powder falling. In other words, the raw thermal insulation material becomes powder and falls due to a lapse of time, vibrations from outside, or the like. Accordingly, a precision device or a device having a moving structure easily receives an adverse effect from the fallen powder. For example, in a case where the thermal insulation material is utilized for the motor, there is a problem in that the powder influences rotation operation of the motor.